Nitrogen oxides (NOx) are major air pollutants contributing to the formation of acid rains and urban smoke with devastating impact on the nature and the health of peoples. Recent environmental EU or North America standards will soon require cleaner vehicles having engines with low emissions of NOx (Euro 6-Euro 7, US EPA standards MY2007 and later), which could not be achieved without advanced after treatment systems with NOx reduction catalysts.
Two basic modern methods to reduce NOx emissions are currently LNT (lean NOx trap, sometimes called as NSR (NOx storage-reduction)) and SCR (selective catalytic reduction) with ammonia. Both methods have their advantages and drawbacks.
LNT-catalysts use rich pulses with diesel fuel as a reductant, storing NOx as nitrates and nitrites on oxides of basic nature like barium oxide under prevailing lean conditions, and reducing stored NOx during short rich pulses on PGM components. LNT is expensive due to high PGM loading, is vulnerable to sulfur, and requires complex desulfurization procedure; current LNT formulations have narrow a temperature window and limited thermal stability.
SCR-catalysts use ammonia, which can reduce more effectively nitrogen oxides NOx in a comproportionation reaction within wide temperature range and under different conditions. An SCR-catalyst does not require PGM components and is therefore less expensive. However, the SCR method needs an additional reductant like ammonia, or more commonly urea, which can decompose to ammonia. Hence, the implementation of an SCR-catalyst requires additional infrastructure and equipment to supply vehicles with urea or ammonia, respectively. Typically, Cu/zeolite or Fe/zeolite is a capable SCR catalyst.
Recently, a new approach with combined LNT-SCR appeared, in which the SCR catalyst is installed downstream of the LNT catalyst. Most of the time the system is operated under prevailing lean conditions, enabling economical running of the engine. During this phase, the NOx emissions are adsorbed by the LNT catalyst. However, short fuel enrichments need to be applied periodically for the LNT regeneration to reduce the stored NOx Ammonia, which is produced in the LNT as a by-product of the NOx reduction under controlled fuel-rich conditions is then adsorbed in the NH3—SCR catalyst located downstream. The adsorbed NH3 is consequently utilized in selective NOx reduction during the next fuel-lean period. The LNT+SCR configuration thus eliminates the need for an external NH3 source (e.g., periodically re-filled urea solution tank) that is necessary in the case of the stand-alone SCR.
Such systems are described as combined LNT-SCR binary catalyst, sometimes also called passive NH3−SCR in U.S. Pat. No. 7,490,464 B2, US 2010/132335 A1, DE 10 2008 043 706 A1, WO 2010/022909 A1, US 2010/050613 A1, US 2010/050604 A1, US 2009/173064 A1, US 2008/282670 A1, US 2008/072575 A1, US 2007/277507 A1, US 2007/271908 A1, US 2007/056268 A1, US 2007/006573 A1, US 2006/010857 A1. Other references in non-patent literature can be found in Laboratory and Vehicle Demonstration of “2nd-Generation” LNT+ in-situ SCR Diesel NOx Emission Control Systems” Lifeng Xu, Robert McCabe, Mark Dearth and William Ruona, Ford Motor Co., Society of Automotive Engineers, SAE 2010-01-030; “Modeling of a combined NOx storage and NH3−SCR catalytic system for Diesel exhaust gas after treatment” Chatterjee, Daniel, Koci, Petr, Schmeisser, Volker, Marek, Milos, Weibel, Michel, Krutzsch, Bernd, Daimler AG, Stuttgart, Germany Catalysis Today (2010), 151(3-4), 395-409; “The Effects of Sulphur Poisoning and Desulfation Temperature on the NOx Conversion of LNT+SCR Systems for Diesel Applications” Joseph R. Theis, Justin A. Ura and Robert W. McCabe, Ford Motor Company, SAE 2010-01-0300; “NOx removal over a double-bed NSR−SCR reactor configuration” Bonzi, R., Lietti, L., Castoldi, L., Forzatti, P. Dipartimento di Energia, Laboratory of Catalysis and Catalytic Processes and NEMAS, Centre of Excellence, Politecnico di Milano, Milan, Italy. Catalysis Today (2010), 151(3-4), 376-385; “Impact of a Cu-zeolite SCR catalyst on the performance of a diesel LNT+SCR system” Xu, Lifeng; McCabe, Robert; Ruona, William; Cavataio, Giovanni Ford Motor Company, USA. Society of Automotive Engineers, [Special Publication] SP (2009), SP-2254 (Diesel Exhaust Emission Control), 121-132; “NOx performance of an LNT+SCR system designed to meet EPA 2010 emissions: results of engine dynamometer emission tests.” Dykes, Erik C. Eaton Corporation, USA. Society of Automotive Engineers, [Special Publication] SP (2008), SP-2217 (Commercial Vehicle Emissions), 57-67; “Transient on-road emission reduction of an LNT+SCR after treatment system” Chimner, Christian Eaton Corporation, USA Society of Automotive Engineers, [Special Publication] SP (2008), SP-2217 (Commercial Vehicle Emissions), 45-56; “Calibration of a LNT-SCR diesel after treatment system” Snow, Rachel; Cavataio, Giovanni; Dobson, Doug; Montreuil, Cliff; Hammerle, Robert. Ford Motor Company Research and Advanced Engineering, Dearborn, Mich., USA Society of Automotive Engineers, [Special Publication] SP (2007), SP-2080 (Diesel Exhaust Emission Control), 377-385; “Ammonia on a LNT: avoid the formation or take advantage of it.” Hackenberg, Stefan; Ranalli, Marco ArvinMeritor Emissions Technologies GmbH, Germany Society of Automotive Engineers, [Special Publication] SP (2007), SP-2080 (Diesel Exhaust Emission Control), 337-345; “Robustness of a LNT-SCR system to aging protocol” Snow, Rachel; Dobson, Doug; Hammerle, Robert; Katare, Santhoji Ford Motor Company Research and Advanced Engineering, Dearborn, Mich., USA Society of Automotive Engineers, [Special Publication] SP (2007), SP-2080 (Diesel Exhaust Emission Control), 127-137; “A LNT+SCR system for treating the NOx emissions from a diesel engine.” Theis, Joseph; Gulari, Erdogan Ford Motor Company, Dearborn, Mich., USA Society of Automotive Engineers, [Special Publication] SP (2006), SP-2022 (Diesel Exhaust), 45-59.
In general, these publications and patent references basically describe the concept of a combined LNT-SCR, typically using commercially available LNT and SCR catalysts, developed for LNT and SCR-alone technologies without their further modification or adaptation for this specific combined LNT+SCR approach. However, current commercial LNT and SCR catalysts are not designed for specific LNT-SCR applications with an SCR catalyst placed downstream of an LNT catalyst. Basic problems are the following: narrow temperature window of NOx conversion, low efficiency of ammonia production on the LNT catalyst, NOx slip during transition from the ECE to EUDC part of the NEDC driving cycle, additional CO production on the SCR catalyst as well as the high cost of the LNT catalyst due to high Pt and Rh loading.
H-Y Chen, E. C. Weigert J. M. Fedeyko, J. P. Cox & P. J. Anderson from Johnson Matthey describe in SAE-2010-01-0302 more advanced catalysts for combined LNT-SCR applications. However, further improvement and adjustment of both LNT and SCR catalyst for combined LNT-SCR applications is necessary, authors from Johnson Matthey also claim the lower cost for LNT, but in fact, the cost reduction is quite low due to the PGM loading of 80 g/ft3 Pt, 20 g/ft3 Pd and 20 g/ft3 Rh. This PGM-loading is still very high, especially the Rh loading is enormous, in the meantime Rh is the most expensive PGM component
According to this background, the underlying objective of the current invention is to present a combination of catalysts, which can be significantly less expensive. In addition, this combination of specially designed catalysts shall improve the performance when used in a combined LNT-SCR system with the SCR catalyst located downstream of the LNT catalyst.
This objective is solved by a LNT-catalyst for an exhaust gas treatment system, in particular to be used as an exhaust gas treatment system for a combustion engine, comprising a mixed metal oxide having proton-conducting properties, and a mixture of at least two platinum group metals (PGM) on a support, wherein the mixed metal oxide is a lanthanum-cerium-oxide, in which up to 40 mol-% of lanthanum is replaced by calcium and/or strontium, and from 0 to 66 mol-% of cerium can be replaced by zirconium and/or praseodymium and the platinum group metals comprise Pt and Pd or Pt—Pd—Rh combination.
The LNT-catalyst is comprising a mixed metal oxide and a mixture of at least two platinum group metals (PGM) on a support and is especially useful to be installed upstream of a SCR-catalyst. A further object of this invention is directed to the SCR-catalyst containing zeolite for an exhaust gas treatment system, in particular to be used in combination with the before mentioned LNT-catalyst. This invention typically deals with a catalytic system comprising a combined LNT-SCR-catalyst.
The invention basically relates to the abatement of nitrogen oxides (NOx) from combustion exhaust gas streams. The invention relates in particular, to a process for reducing NOx from oxygen containing exhaust gases from internal combustion engines (ICE), in particular emitted from diesel engines or lean-burn gasoline engines and could be also related to a removing process of NOx formed by any other combustion processes, such as in stationary engines, in industry, etc.
It has been found that a catalyst of such composition can highly effectively produce ammonia and reduce NOx while at the same time having a lower PGM-loading compared to those catalysts currently known. Because of these characteristics, such a catalyst is especially useful as a LNT-catalyst upstream of a SCR-catalyst.
According to a preferred embodiment of the LNT-catalyst of this invention, the mixed metal oxide has defect fluorite structure with proton-conducting properties.
It has been found to be advantageous, if up to 66 mol-% of cerium is replaced by zirconium or praseodymium. Such catalysts can provide an even higher ammonia production rate.
Although the LNT-catalysts of this invention also work with higher PGM-loadings, it has been found that the total loading of platinum group metals of 100 g/ft3 or less is sufficient for a highly effective production rate of ammonia in the relevant temperature range of exhaust gases, i.e. from about 150° C. to about 450° C.
According to the preferred embodiment of the LNT-catalyst of this invention, the Pt-loading is 10-70 g/ft3, preferably 25-35 g/ft3.
A further preferred LNT-catalyst according to this invention is characterized by a Pd-loading of 5-40 g/ft3, preferably 10-15 g/ft3.
Another preferred embodiment is Rh-loading up to 10 g/ft3, in particular 2-5 g/ft3 is highly preferred as these loadings provide a good compromise between catalytic activity and costs of the catalyst.
The LNT-catalyst may comprise any suitable support, which is typically used in exhaust gas after treatment systems. Preferably however, the support is selected from cordierite, in particular provided with an alumina wash coat.
The production of LNT-catalyst according to this invention may be carried out by a process method including impregnation of the support with a solution of La, Ce, Ca, Sr, Zr, Pr salts, respectively, in particular with an aqueous solution of the nitrate salts further containing urea; drying and afterwards calcinations at temperature of at least 850° C. for at least 1.5 h, in particular using the drying at 88° C. for about 10 h, then drying at 100° C. and preferably further calcinations at 900° C. for 2 h; impregnation of the modified support with a solution of the platinum group metals precursors, in particular with an aqueous solution of non-chlorine salts of these metals further containing citric acid and urea; drying and afterwards calcinations of the PGM-impregnated support at temperature of at least 550° C. for at least 1.5 h, in particular drying at a 88° C. for about 10 h, then drying at 100° C. and calcinations at 600° C. for 2 h.
Another object of this invention is directed to a SCR-catalyst for an exhaust gas treatment system, in particular to be used in combination with an LNT-catalyst according to this invention for selective catalytic reduction of nitrogen oxides with ammonia in an exhaust gas treatment system for a combustion engine, the SCR-catalyst being prepared by a method including modifying a zeolite containing support by essential removal of aluminum extra-lattice/extra-framework species from the zeolite; calcinations of the modified support at a temperature of at least 550° C., in particular at 600° C.; and insertion of Cu, Cu/Ce, Mn/Ce or Co/Ce into the modified zeolite by ion-exchange and/or impregnation.
It has been found that such SCR-catalyst is highly effective in combination with the LNT-catalyst of this invention because the catalytic activity of such an SCR-catalyst in terms of NOx-conversion with ammonia is very high in the same temperature window, in which the LNT-catalyst of this invention has its peak effectiveness for ammonia production. Accordingly, these catalysts are connected to each other by having the same optimal operating conditions in an exhaust gas after treatment system.
Preferably, the support of such an SCR-catalyst is a Cu/zeolite including a commercial Cu/zeolite catalyst on cordierite or another support, whereas the removal step mentioned above comprises essential removal of copper and aluminum extra-lattice/extra-framework species from the commercial zeolite.
According to a further embodiment of the SCR-catalyst according this invention, the removal of Cu and/or Al is carried out with a citrate-containing solution, in particular comprising citric acid and diammonium hydrogen citrate, whereas the removal and calcination steps are repeated at least once.
For the SCR-catalyst it is even more preferred, if the first step is performed at a temperature of 80-100° C., preferably of about 97° C.
The SCR-catalyst of this invention may further be characterized in that the insertion step is carried out in the presence of urea, in particular using an aqueous solution with nitrate salts of Cu, Ce, Mn and Co, respectively.
A highly preferred SCR-catalyst of the current invention is characterized in that the insertion step is followed by drying for a period of at least 6 hours, in particular at a temperature of about 88° C., and calcinations at a temperature of at least 550° C., preferably at about 600° C.
The SCR-catalyst of this invention may be deposited on any suitable support, which is typically used in exhaust gas after treatment systems. Preferably however, the support is cordierite, in particular provided with a zeolite washcoat.
Another object of this invention is a catalyst system for an exhaust gas treatment system, in particular to be used as an exhaust gas treatment system for a combustion engine, comprising an LNT-catalyst of this invention and an SCR-catalyst, wherein the SCR-catalyst is positioned downstream of the LNT-catalyst. According to this embodiment, any commercially available SCR-catalyst can be combined with the LNT-catalysts of this invention. It is however preferred, if the SCR-catalyst is an SCR-catalyst of this invention. The reason for this is that both catalysts of this invention work together very well due to their similar catalytic activity profile over the relevant temperature range from 150° C. to 450° C. Further, the SCR catalyst of this invention has additional advantages relative to common SCR Cu/zeolite catalyst for stand-alone SCR applications, namely higher ammonia storage capacity, better NOx reduction with CO and HC presented in Diesel exhaust, Ce-containing SCR catalysts of invention have also additional NOx storage and better CO oxidation properties than common Cu/zeolite SCR catalyst. These features of new developed LNT and SCR catalysts provide more effective NOx emissions reduction for combined LNT-SCR applications and offer more cost effective solution for lean NOx Diesel after treatment.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
In the following, the invention is presented in part with preferred embodiments and FIGS. 1 to 11 in more detail. The figures show: